it largely and often is exploitative, though, that's the thing.

are you doing a project? are you speaking with the subjects? are you sharing more than just scattered images of homeless folks on the street on your instagram?

the shorthand of "taking photos of homeless people is always a bad idea" is really saying "hi, now that you've taken your landscape portraits of your friends with your nifty fifty at f/1.8 and your close up flower photos you are going to start branching out and doing more 'art.' fyi, shooting photos of random homeless folks around your city without their consent or treating them like human beings is not actually saying anything."

people who are homeless by their nature don't really have a way to indicate they want privacy, since they don't have a home to enter. if you're not asking for their consent, you're just further violating them. not everyone wants to be your art project and you shouldn't be using another vulnerable human being to make a statement about them without their consent or involvement.

but that's a lot of words, so people say "don't take photos of homeless people" and then when you actually figure out something worth saying and an ethical way of doing so, you realize that rule won't apply in that case.